Custom Render Stamp, Principled BDSF values
I found a solution with various references.  The big thing that helped was finding how to enter the value for a Principled BSDF parameter in the console;

bpy.data.materials['Material'].node_tree.nodes["Principled BSDF"].inputs[1].default_value = 3 


Mostly from here:

Change value of material emission strength from Python console or script

The final script looks like this (input 7 is Roughness):

import bpy

def stamp_set(scene):
    note  =   "Samples: "                     + str(scene.cycles.samples)
    note += ", Bounces, Max: "                + str(scene.cycles.max_bounces)
    note += ", Roughness: "                + str(round(bpy.data.materials['Material'].node_tree.nodes["Principled BSDF"].inputs[7].default_value, 3))
    note += ", Version: "                     + bpy.app.version_string
    scene.render.stamp_note_text = note


bpy.app.handlers.render_pre.append(stamp_set)


ref:

Editing Lamp Strength with Python

Change value of sun light emission strength from Python console or script

decreasing excess precision:

How to show render-stamp for arbitrary values?

-snaok